Filter devices of this type are prior art in a wide range of designs. In hydraulic systems, the return flow from working hydraulics, for example, in the form of working cylinders, is filtered in an open hydraulic circuit. From a closed hydraulic circuit of the associated hydrostatic drive, such as pumps or motors, oil leakage flows incur. Problems arise in connection with the filtration of the resulting leak-oil. In view of maintaining the required system purity, the filter element of the working filter, filtering the return flow from the working hydraulics, has to be designed with a high grade of filtration to ensure microfiltration of the return flow. If the return flow is high volume, if the hydraulic fluid is high viscosity (cold conditions) or in case of considerable contamination of the filter element, relatively high back pressures result at the working filter. This high pressure precludes using the working filter designed for large filtration grade for superfine filtration of resulting leak-oil, as the leak-oil producing components, such as pumps or motors, do not tolerate higher leak-oil pressures because of their shaft bushings and gaskets. Provisions must then be made for leak-oil filtration, for example, separate leak-oil filters of a coarser filtration grade, to avoid higher back pressures, which in turn has negative effects on system purity.